This invention relates to a phase shift mask capable of improving a resolution of a transfer pattern by imparting a phase difference to exposure light beams passing through the mask and to a phase shift mask blank from which the phase shift mask is made. More particularly, this invention relates to a phase shift mask of a so-called halftone type and a phase shift mask blank from which the phase shift mask is made.
To manufacture a semiconductor LSI, use is made of a phase shift mask which is known as one of photomasks for transferring a fine pattern. The phase shift mask imparts a phase difference to exposure light beams passing through the mask to thereby improve a resolution of a transfer pattern. As one of those phase shift masks of the type described, Japanese Patent Publication (A) H04-136854 discloses a phase shift mask which is particularly adapted to transfer an isolated pattern such as a single hole, dot, or line.
In the phase shift mask disclosed in the above-referenced publication, a mask pattern formed on a transparent substrate comprises a light transmitting portion and a light translucent portion. The light transmitting portion transmits effective light beams having an intensity which substantially contribute to exposure. On the other hand, the light translucent portion transmits ineffective light beams having an intensity which does not substantially contribute to exposure. The ineffective light beams passing through the light translucent portion are shifted in phase so that the ineffective light beams passing through the light translucent portion have a phase substantially reverse to that of the effective light beams passing through the light transmitting portion. In this arrangement, the effective and the ineffective light beams passing through an area in the vicinity of a boundary between the light transmitting portion and the light translucent portion cancel each other. It is therefore possible to assure an excellent contrast at the boundary. The phase shift mask described above is called a halftone type. In this phase shift mask, the light translucent portion has both a light shielding function of substantially shielding the effective light beams and a phase shift function of shifting the phase of the ineffective light beams. Accordingly, it is unnecessary to separately form a light shielding film pattern and a phase shift film pattern. Thus, the phase shift mask is simple in structure and easy in manufacture.
In the meanwhile, the light translucent portion in the above-mentioned phase shift mask of a halftone type is required to have optimum values for both a light transmissivity and phase shift ability. If the light translucent portion is formed by a single material, both of the above-mentioned requirements must simultaneously be fulfilled by selecting its thickness. However, an appropriate material fully satisfying such requirements has not yet been developed so far.
In view of the above, a proposal is made of an improved light translucent portion of a multilayer structure comprising a plurality of kinds of materials in the form of a high-transmissivity layer and a low-transmissivity layer. The low-transmissivity layer mainly serves to adjust the light transmissivity to a predetermined value. On the other hand, the high-transmissivity layer mainly serves to adjust an amount of the phase shift. With this structure, it is readily possible to adjust both the light transmissivity and the phase shift amount to optimum values. FIGS. 1 and 2 show phase shift masks each of which has a light translucent portion of a multilayer structure. Referring to FIG. 1, the phase shift mask comprises a transparent substrate 10, a stopper film 11 formed on the transparent substrate 10, a SOG (spin on glass) film 12 formed on the stopper film 11 to serve as the high-transmissivity layer, and a chromium film 13 formed on the SOG film 12 to serve as the low-transmissivity layer. Referring to FIG. 2, the phase shift mask comprises the transparent substrate 10, the chromium film 13 formed on the transparent substrate 10, and the SOG film 12 formed on the chromium film 13.
However, the above-described phase shift masks with the light translucent portions of a multilayer structure have following disadvantages.
As described above, the high-transmissivity layer and the low-transmissivity layer are made of SOG and chromium, respectively. Therefore, when etching is carried out to form a mask pattern, it is required to use different kinds of etching media for the SOG film 12 and the chromium film 13 as the high-transmissivity layer and the low-transmissivity layer, respectively. It is assumed that both the SOG film 12 and the chromium film 13 are subjected to dry etching in order to suppress occurrence of side etch. In this event, the SOG film 12 as the high-transmissivity layer is etched by the use of a series of fluoride etching gases such as CF.sub.4, CHF.sub.3, SF.sub.6, C.sub.2 F.sub.6, NF.sub.3, CF.sub.4 +H.sub.2, CBrF.sub.3. On the other hand, the chromium film 13 as the low-transmissivity layer is etched by a series of chlorine gases such as CCl.sub.4 and Cl.sub.2. If both of the films are successively etched in a same etching apparatus, one etching gas used earlier may remain in the apparatus to be mixed with the other etching gas used later. This possibly results in disturbance of etching conditions. In order to eliminate the possibility of such disturbance, it is proposed to etch those films in separate etching apparatuses. In this event, however, an additional etching apparatus is required and the substrate must be moved from one etching apparatus to the other. Furthermore, during the movement, foreign particles such as dust may be attracted onto the substrate to cause a defective pattern to be formed. In addition, the SOG film has a low refractive index and must therefore be relatively thick. This means that the mask pattern has a large step height. As a result, the mask pattern is readily damaged during washing and washability is insufficient to remove the foreign particles attracted thereto.